In order to effectively build circuits on p-type Hg.sub.1-x Cd.sub.x Te, electrical contacts to the substrate must be established. Ohmic contacts to p-type Hg.sub.1-x Cd.sub.x Te have proven difficult because most common contacting processes either convert the semiconductor to n-type, thereby creating a p-n junction diode in the p-type material or result in a Schottky diode. Ohmic contacts to p-type Hg.sub.1-x Cd.sub.x Te have typically been made by the deposition of gold using either electroless plating in a gold solution or by thermal evaporation These contacts prove less than durable. Attempts have also been made to use copper, platinum, germanium, aluminum, titanium, HgTe and small-bandgap Hg.sub.1-x Cd.sub.x Te.
While contacts using gold as the contact metal (particularly electroless gold) can be ohmic and of low resistance, they suffer from a mechanically weak gold to Hg.sub.1-x Cd.sub.x Te interface, which causes poor adhesion of gold film. These films often delaminate during the processing that is required to build a multilayer integrated circuit. Even if gross delamination does not occur, these gold contacts are plagued by non-reproducible contact resistances which may also result from a lack of integrity at the gold to Hg.sub.1-x Cd.sub.x Te interface. Platinum, germanium, aluminum and titanium produce rectifying and therefore highly resistive contacts. Copper is an acceptor in Hg.sub.1-x Cd.sub.x Te and is known to diffuse quickly at room temperature. Even small amounts of copper in contact with Hg.sub.1-x Cd.sub.x Te have the potential to raise the acceptor concentration of the entire slice to levels which will dramatically degrade Metal Insulator Semiconductor (MIS) performance.
High quality films of small-bandgap Hg.sub.1-x Cd.sub.x Te and HgTe grown by molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD) have been shown to produce ohmic contacts to p-type Hg.sub.1-x Cd.sub.x Te, but require expensive and complex deposition equipment and deposition temperatures which are problematically high compared to Hg.sub.1-x Cd.sub.x Te processing temperatures. Selective area deposition by MBE or MOCVD on Hg.sub.1-x Cd.sub.x Te is difficult if not impossible.
HgTe contacts formed using thermal evaporation of HgTe onto a 5.degree. C. substrate requires considerable thermal annealing to become ohmic and does not achieve specific contact resistances as low as tin contacts.
A long felt need therefore continues to exist for an improved contact to a p-type semiconductor material containing mercury.